Process for manufacture of tamper-resistant polyester credit cards

ABSTRACT

The present invention relates to a credit card or identification card fabricated from amorphous polyester sheet material wherein at least the area of said card containing embossed numerical or letter characters and the characters themselves are composed of substantially crystalline polyester. Such characters can not readily be altered by either heat treatment or by shaving them off.

BACKGROUND OF THE INVENTION

The fraudulent use of credit cards is becoming more prevalent everyyear. Credit card blanks are most commonly fabricated from plasticmaterial and comprise a core sheet, usually polyvinyl chloride orcopolymers thereof, upon which certain information concerning the issuerof the credit card is printed. The printed core may be overlayed with avinyl film on one or both sides and the film is heat laminated to thecore stock. Adhesives are commonly used to provide for firmer bonding ofthe core stock to the overlay. An example of such a laminated structureis set forth in U.S. Pat. No. 4,133,926.

It is also known to provide identification cards which have beenspecifically devised to be resistant to counterfeiting. One suchstructure comprises a laminate of a polyester film layer laminated to auniaxially oriented plastic by means of an adhesive layer, as disclosedin U.S. Pat. No. 4,343,851. Other such identification cards aredisclosed in U.S. Pat. No. 4,325,196.

As is well known, credit cards also contain embossed charactersidentifying the name and account number of the credit card holder. Thesecharacters are offset and raised such that the information may betransferred to charge slips. In conventional practice, printing isapplied directly to the plastic core sheet, while embossing is effectedafter the core sheet has been laminated between the vinyl films.

Unfortunately, the embossed characters on lost or stolen credit cardsmade from the conventional thermoplastic materials can be easilyaltered. One or more of the embossed characters can be shaved off thesurface of the card and a different character pasted on. Thus two digitsof an account number can be transposed giving an entirely differentnumber. Embossed names can be altered or shortened using the sametechnique.

Still another technique for altering credit card characters is to simplyiron them out. Thermoplastic material can be made to flow by theapplication of pressure and sufficient heat thereby removing theembossed characters. New characters may then be embossed by simplystamping them at the appropriate location on the card.

If these and similar alteration techniques are done carefully enough, itmay be very difficult to differentiate between an altered card and anoriginal.

Accordingly, it is an object of this invention to provide a plasticcredit card having embossed characters which can not readily be alteredor removed.

Yet another object is to provide a plastic credit card which can notreadily be re-embossed with altered characters.

SUMMARY OF THE INVENTION

These and other objects may be achieved by the utilization ofsubstantially amorphous polyester sheet material as core sheet in thefabrication of credit card blanks and by embossing such sheet materialor blanks with the desired characters followed by the application ofsufficient heat or other means to substantially crystallize theamorphous polyester core sheet at least in that area where the embossedcharacters are formed. In the resulting article and because of theproperties of crystalline polyester, the embossed characters can not beeasily shaved off without destroying the shape of the character itself.If one attempts to iron the characters out, there remains a "plasticmemory" of the characters as originally embossed such that significantevidence of the original characters is discernible on the surface of thearticle. In addition, it becomes difficult to re-emboss new numbers onthe now-crystalline polyester core sheet surface.

Actually, any attempt to heat alter the crystalline polyester portionsof the article would require temperatures so high (in excess of 450° F.)that the article could be distorted or disfigured, thereby making itmore obvious that an attempt had been made to alter the article.

DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, there is illustrated a schematic sectional view ofa credit card structure 1 comprising a sheet 2 of polyester. The sheethas been embossed to produce offset characters 9.

FIG. 2 depicts a schematic sectional view of a laminated structure suchas a credit card identified by reference numeral 10. The structureincludes a core sheet 12 formed of polyester sheet. The upper surface ofthe illustrated sheet is partially covered with printing ink 14. Outeroverlay films 18 are laminated to both surfaces of the polyester sheet.Layer 16 represents the dry residue of an adhesive used to improve theadhesion between film 18 and sheet 12. The sheet has been embossed toproduce offset characters 19. In each of these drawings the thickness ofthe card structure has been exaggerated for the purposes of clarity. Inpractice, the height and depth of the male/female embossements 9 and 19would be about the same as the thickness of the card structure.

DETAILED DESCRIPTION OF THE INVENTION

Amorphous polyester core sheet material useful for the purposes of thisinvention may be formed from any of the well known thermoplastic heatcrystallizable polyesters produced by condensing an aromaticdicarboxylic acid or a lower alkyl diester thereof with a glycol. Amongsuch dicarboxylic acids are included terephthalic acid and copolymersthereof with minor amounts of isophthalic; phthalic; succinic; sebacic;adipic; azelaic; bibenzoic or like acids. One or more of these acids oracid esters is reacted with one or more glycols which include ethyleneglycol; 1,3-propanediol; 1,4-butanediol; neopentyl glycol or1,4-cyclohexanedimethanol. The preferred film forming polyester usefulfor the purposes of this invention is polyethylene terephthalate.

The amorphous polyester sheet may be formed by heating the polyester toa temperature above its melting point and continuously melt extrudingthe material in sheet form through a slot die onto a chilled castingdrum after which it solidifies into a substantially amorphous sheet. Theamorphous sheet may be then cooled and rolled. The preferred thicknessof the amorphous sheet for credit card applications is in the range ofabout 5 to 40 mils, so the extrusion die slit opening should be gaugedto provide sheet within such range of thickness.

The core sheet material may be overlayed on one or both surfaces with athinner gauge film material which serves to both protect matter printedon the core sheet and to give the article a better appearance. Virtuallyany plastic film material which can be laminated with good adhesion tothe amorphous polyester core sheet can be employed, e.g., polyvinylchloride film, but it is preferred to utilize mono or bi-axiallyoriented polyester film for this purpose. Such polyester film may beprepared from the same monomers as set forth above with respect to thecore sheet and the oriented film is manufactured as is known in the artby stretching the film in one or two mutually perpendicular directions,as for example as set forth in U.S. Pat. No. 4,214,035. The preferredfilm is biaxially oriented crystalline polyethylene terephthalate filmhaving a preferred thickness in the range of about 0.25 to 2 mils, morepreferably about 1 mil. The use of the overlay film is optional, butpreferred.

In many cases it may be desirable or necessary to utilize an adhesivematerial to improve the laminate adhesion between the polyester coresheet and the polyester overlay. This is particularly important wherethe core sheet has been printed with wet or dry lithographic offset inksor screen printing inks, since the ink will tend to interfere with goodlamination. Adhesive materials which can be used include acrylic ormethacrylic resins, ethylene/vinyl acetate copolymers, water dispersiblecopolyesters containing at least one free functional acid group in thepolymer chain such as disclosed in U.S. Pat. Nos. 4,252,885 and4,304,851, incorporated herein by reference, and heat sealable coatingpolymers such as disclosed in British Pat. No. 1,078,813. The adhesivemay be applied either to the surface of the polyester core sheet or tothe surface of the overlay film prior to lamination.

Preferably the adhesive material is applied to the surface of theoverlay film either during or after its manufacture and the core sheetmaterial may then be laminated to the coated surface of one layer, orbetween the coated surfaces of two layers, of the overlay film.Lamination may be accomplished using a heat press having highly polishedmetal surfaces. The thickness of the adhesive layer on the orientedpolyester film is preferably within the range of about 10⁻⁷ to 2×10⁻⁴inch.

The amorphous core sheet and/or protective film used in the presentinvention may contain conventional additives as known in the artincluding filler materials, such as silica, talc, calcium chloride, orclay and/or colorants, such as titanium dioxide, as well as otherpigments.

Credit card blanks are generally fabricated by printing with adherableinks on the core sheet material the desired commercial information, suchas by screen or offset methods, and subsequently laminating theretounder heat and pressure an overlay film on one or both sides. Thecomposite is then embossed under pressure and in some cases heat withnumerical and/or letter characters, either before or after which saidcomposite is cut into credit card size.

In its broader aspect, credit cards of this invention may be prepared by(1) providing a supply of amorphous polyester sheet; (2) embossing saidsheet (either before or after cutting said sheet into the appropriatesize for a credit card) under pressure and optionally heat to formoffset characters on the card; (3) heating said card for a time and at atemperature sufficient to crystallize the amorphous polyester, or atleast that portion of the card bearing offset characters whilepreferably maintaining said card under restraint to prevent any curlingor distortion of the card; and (4 ) cooling said card to ambientconditions.

Credit cards of this invention are also prepared by: (1) providing asupply of amorphous polyester sheet, commercially printed using apolyester screen ink adherable to the polyester surface; (2) providing asupply of overlay film, optionally coated on at least one side with anadhesive for enhancing the lamination bond between said overlay film,said ink and the printed surface of said polyester sheet; (3) laminatingby heat and pressure the overlay film to both sides of the polyestersheet for a time and at a temperature sufficient to provide good laminaradhesion, but insufficient to cause substantial crystallization of theamorphous sheet, to provide a composite with a tough durable overlay;(4) cutting the composite into the right size and shape for a creditcard, i.e., about 31/4 to 31/2 inch by 2 to 2 1/4 inch; (5) embossingsaid card under pressure, and optionally heat, with numerical and/orletter characters using an appropriate device such as an offset press orlabel maker to form offset characters on the card; (6) heating said cardfor a time and at a temperature sufficient to crystallize the amorphouspolyester card, or at least that portion of the card containing theoffset characters while preferably restraining said card to prevent anycurling or distortion thereof, and (7) cooling the card to ambientconditions.

In the embodiments referred to above, the temperature required toachieve partial or total crystallization of the amorphous polyestersheet or card may vary as a function of time in that crystallization isboth time and temperature dependent. Generally, temperatures in therange of 90° C. to 240° C. may be employed, with the maximum rate ofcrystallization occurring at about 165° C. At any given temperature, thetime required for crystallization will be evident by observation andevaluation of the change in properties of the polyester. Any source ofenergy which will provide sufficient heat to crystallize the amorphouspolyester or portion thereof bearing the offset characters may be used.This would include infrared and microwave sources.

In the preferred embodiment where the amorphous polyester sheet islaminated to one or two overlay films, the temperature required toachieve good adhesive bonding will vary depending on the properties ofthe overlay film and whether or not an adhesive layer is present and thenature of the printing ink present on the surface of the amorphouspolyester sheet. Temperatures sufficient to soften the adhesive layerare required, and the pressure must be sufficient to form a good firmbond between the layers, i.e., greater than about 25 psi. Temperature,pressure and time conditions must, however, not be such that anysubstantial crystallization of the amorphous polyester sheet ispermitted to occur.

The following Examples are illustrative of the invention:

EXAMPLE 1

Two strips of biaxially oriented polyethylene terephthalate film havinga thickness of about 0.48 mil were laminated to both sides of a strip ofa cast sheet of amorphous polyethylene terephthalate having a thicknessof about 7 mils. Lamination was accomplished using a gradient bar sealerat a temperature of about 300° F., a pressure of 80 psi and for a periodof about six seconds. The resulting composite was then embossed at roomtemperature with various numerical and letter characters using a Dymo®label maker. The resulting embossed composite was mounted in a framerestraint to prevent heat distortion and then heated in a convectionoven at 125° C. for a period of 10 minutes to crystallize the amorphouscore sheet.

EXAMPLE 2

A strip of cast amorphous polyethylene terephthalate film having athickness of 6.9 mils was placed in a letter vise and embossed at roomtemperature with lettering by closing the vise. The vise containing therestrained film was placed in a convection oven at 125° C. for about 10minutes to crystallize the polyester. After 10 minutes the assembly wasremoved from the oven and allowed to cool to room temperature beforeopening the vise.

EXAMPLE 3

The procedure of Example 1 was repeated except that the strips ofbiaxially oriented polyethylene terephthalate film were coated with anaqueous adhesive layer comprising a copolyester having free sulfo acidgroups in the polymer chain and dried. The strips were laminated to thecore using this adhesive layer to promote adhesion.

X X X

The strips prepared according to Examples 1 and 2 were evaluated fortheir resistance to tampering in comparison with two control samples.Control A was a 10.7 mil thick strip of unplasticized polyvinyl chloridesheeting; Control B was a 7.0 mil thick strip of biaxially oriented(crystallized) polyethylene terephthalate sheeting. Each of thesematerials were embossed at room temperature with various numerical andletter characters by the method set forth in Example 1.

The resistance of the embossments in each sample to "ironing out" wasevaluated using a laboratory gradient heat sealer set at 300° F. Theheat sealer was applied to the embossed strips at a pressure of 50 psifor a period of six seconds.

The resistance of the embossments to "shaving" was evaluated byattempting to shave them off the surface of the sheet using an ordinaryrazor blade.

Results are recorded in the Table.

                  TABLE                                                           ______________________________________                                        Ironing Out         Shaving                                                   ______________________________________                                        Ex 1   Some flattening, but                                                                           Embossments could not                                        obvious evidence of                                                                            easily be removed.                                           the embossments is                                                            apparent.                                                              Ex 2   Some flattening, but                                                                           Embossments could not                                        obvious evidence of the                                                                        easily be removed.                                           embossments is apparent.                                               Control                                                                              Severe flattening                                                                              Characters could be easily                            A      but faint evidence                                                                             cut from sheet without                                       of the embossment.                                                                             destruction of their                                                          shape.                                                Control                                                                              Embossment was flat                                                                            Characters were not                                   B      due primarily to the                                                                           high enough to be                                            initial resistance                                                                             effectively removed                                          to embossing of  by a razor blade.                                            crystalline polyester.                                                 ______________________________________                                    

From the above it can be concluded that credit cards prepared byembossing polyester sheet material in the amorphous state followed bycrystallization of the embossed sheet produces a credit card having asuperior resistance to tampering.

What I claim is:
 1. A method of preparing a credit or identificationcard comprising:(a) providing a supply of substantially amorphouspolyester sheet material; (b) embossing a surface of said sheet materialunder pressure with numerical and/or letter characters; (c) holding saidsheet material under restraint while heating said sheet material to atemperature and for a time sufficient to substantially crystallize atleast the embossed portion of said amorphous polyester sheet; and (d)cooling said sheet material.
 2. The method of claim 1 wherein saidamorphous polyester is polyethylene terephthalate.
 3. The method ofclaim 2 wherein said heating temperature is within the range of about90° C. to about 240° C.
 4. The method of claim 3 wherein said sheet hasa thickness within the range of about 5 to 40 mils.
 5. The method ofclaim 4 wherein said sheet material is cut into a size suitable for useas a credit card before or after said embossing step.
 6. The method ofclaim 5 wherein said sheet material contains an opaque filler orcolorant.
 7. A method of preparing a credit or identification cardcomprising:(a) providing a supply of substantially amorphous polyestersheet material; (b) providing a supply of overlay film; (c) laminatingsaid overlay film to one or both sides of said sheet material under heatand pressure insufficient to substantially crystallize said amorphouspolyester sheet to provide a composite sheet; (d) embossing a portion ofa surface of said composite sheet under pressure with numerical and/orletter characters; (e) holding said composite sheet under restraintwhile heating said composite sheet to a temperature and for a timesufficient to substantially crystallize at least the embossed portion ofsaid amorphous polyester sheet; and (f) cooling said sheet material. 8.The method of claim 6 wherein said polyester sheet material is amorphouspolyethylene terephthalate.
 9. The method of claim 7 wherein saidoverlay film is biaxially oriented polyethylene terephthalate.
 10. Themethod of claim 9 including the step of interposing an adhesive layerbetween said amorphous sheet and said overlay film prior to lamination.11. The method of claim 10 wherein said adhesive layer comprises a waterdispersible copolyester having at least one free functional acid groupin the polymer chain.
 12. The method of claim 11 wherein saidcrystallizing temperature is within the range of about 90° C. to about240° C.
 13. The method of claim 12 wherein said crystallizingtemperature is about 125° C.
 14. The method of claim 9 including thestep of cutting said composite sheet material into a size suitable foruse as credit card before or after said embossing step.
 15. The methodof claim 14 wherein said amorphous sheet material has a thickness in therange of about 5 to 40 mils, and said overlay film has a thickness inthe range of about 0.25 to about 2 mils.